As a process for producing a multilayer wiring board, the following process is general. A material, which is prepared by impregnating glass cloth with an epoxy resin and semi-curing the resin (called prepreg), and a copper foil are stacked on an insulating substrate having a circuit pattern formed, and they are laminated together by means of a hot press. Then, a hole for interlayer connection is formed in the resultant laminate by means of a drill, and subsequently the inner wall of the hole and the surface of the copper foil are subjected to electroless plating and, if necessary, further subjected to plating to form a conductor layer having a required thickness. Then, unnecessary copper is removed to form a circuit pattern.
In recent years, electronic apparatus are being further reduced in size, reduced in weight, and increased in function. In accordance with this tendency, the integration degree of LSIs or chip parts is increased, and their form is rapidly changed to one having many pins or a reduced size. Therefore, for increasing the mounting density of electronic parts, the development of refined wiring on a multilayer wiring board is progressed. As a method for producing a multilayer wiring board which meets the demands, there is a build-up process in which no glass cloth is used and an insulating resin composition is used instead of a prepreg to form an insulating layer, and interlayer connection is made only for a required portion through a via hole to form a multilayer structure. This process is being mainly used as a method which meets the demands of weight reduction, size reduction, and refining.
With respect to the insulating resin composition used in the build-up process, for example, there is an adhesive film having excellent circuit filling properties (see, for example, Japanese Unexamined Patent Publication No. 87927/1999). In addition, a resin composition having excellent handling properties when it is in a semi-cured state and having excellent flame retardancy has been disclosed (see, for example, Japanese Unexamined Patent Publication No. 256537/2000).
In the production of a multilayer wiring board, when an insulating layer is formed from an insulating resin composition without using glass cloth, the mechanical physical properties of the insulating layer largely affect the properties of the multilayer wiring board. Specifically, when the insulating layer is rigid and has a small elongation and brittle properties, crack or breakage is caused in the insulating layer during punching out of the multilayer wiring board into product size, which considerably adversely affects the conduction or insulation reliability. For improving the insulating layer in elongation, generally, a method in which a thermoplastic resin having a high molecular weight is introduced into the insulating resin composition is employed. However, the introduction of such a resin poses a problem in that the glass transition temperature of the resultant resin composition is lowered, and in that the coefficient of thermal expansion is increased or the electric properties become poor.
In addition, when using no glass-cloth, the insulating layer is likely to suffer large thermal expansion and a difference is caused between the insulating layer and a base material, a conductor (copper), or solder with respect to the coefficient of expansion, so that crack may be caused in the insulating layer, thus considerably lowering the connection reliability. A generally required coefficient of thermal expansion of the insulating layer is at a level of 50×10−6/° C. or less (average value in the range of 30 to 100° C.).
Further, the data processing rate is currently increased, and hence a lowering of the transmission loss is further important. Currently, the amount of data in a high frequency region is especially increased, and therefore an insulating resin which can exhibit a permittivity of 3.5 or less and a dielectric loss of 0.015 or less in a 1 GHz region is needed.
There has been currently desired an insulating resin composition having the above-mentioned performance, namely, insulating resin composition which is cured to form an insulating layer which simultaneously satisfies, firstly, such a large elongation that the insulating layer can endure mechanical or thermal stress concentration, secondly, such a low coefficient of thermal expansion that the connection reliability can be maintained even after the repeated use at low and high temperatures, and, thirdly, such excellent electric properties that the permittivity and dielectric loss are low.
In addition to the above-mentioned performance, the insulating resin composition is recently required to form an insulating layer having excellent bonding properties to a conductor layer. In the refining of wiring, from the viewpoint of achieving excellent etching accuracy upon forming a circuit, it is preferred that the roughness of the interface through which the insulating layer and the conductor layer are bonded is smaller. A low profile foil has currently been practically used in which the roughness of the roughened copper foil is lowered from a conventional value of between 7 to 8 μm (Rz) to between 3 to 4 μm (Rz). However, such a low profile foil is likely to be lowered in the bond strength to the conductor layer and insulating layer. Therefore, an insulating resin composition which can exhibit high bond strength even to a low profile foil is desired. Furthermore, for preventing environmental problems, the insulating resin composition is required to have excellent flame retardancy although it is free of a halogen. Generally, the performance of a non-halogen flame retardant is often not as good as that of a halogen flame retardant, such as a bromine flame retardant. However, on the other hand, when the flame retardancy is obtained by increasing the amount of the non-halogen flame retardant added to the insulating resin composition, there is a danger that other properties may be adversely affected.
Studies have been made with a view toward solving the above problems. As a result, there has been found an insulating resin composition which comprises (A) a novolak epoxy resin having a biphenyl structure, (B) carboxylic acid-modified acrylonitrile butadiene rubber particles, (C) a triazine ring-containing cresol novolak phenolic resin, (D) a phenolic hydroxyl group-containing phosphorus compound, and (E) inorganic filler. It has been found that the insulating resin composition is advantageous not only in that it exhibits flame retardancy although it is free of a halogen, but also in that a cured film formed from the insulating resin composition realizes such a large elongation that the film can endure stress concentration caused by a mechanical or thermal factor and a coefficient of thermal expansion equivalent to that of a structure containing a reinforcing material, such as glass cloth, and further exhibits excellent high frequency properties.